Heart-lung machines conventionally include a venous reservoir which receives the patient's blood at a variable rate during open-heart surgery and releases it at a substantially steady rate to the oxygenation circuit from which it is returned to the patient. In the operation of the heart-lung machine, it is important for the perfusionist to be continuously advised of the exact volume of blood in the reservoir, as this information is needed to maintain the correct diluted blood volume in the patient and to calculate the proper doses of infused drugs.
Rigid hard-shell reservoirs lend themselves well to this purpose because accurate graduations can readily be inscribed on their surface. However, because the volume of the hard-shell reservoir itself is constant, it will discharge potentially lethal air into the blood circuit of the heart-lung machine if it is allowed to become empty.
Collapsible soft-shell reservoirs (i.e. plastic bags) have the advantage of increasing and reducing their volume in accordance with the amount of blood they contain, and they consequently need no airspace that could produce emboli. On the other hand, soft-shell reservoirs, because they are always exactly filled with blood, cannot provide a visible volume indication by way of graduations.
In the past, perfusionists have estimated the blood volume in soft-shell reservoirs by the appearance of the reservoir bag, but this requires experience and is not sufficiently accurate for modern requirements. To remedy this deficiency, it has previously been proposed to position the reservoir bag between two parallel plates which are biased against the bag, and whose distance from each other as the bag expands and contracts is indicated by a tape measure. That system, however, is not very accurate and is awkward to observe. Ultrasonic volume measurement has been proposed in the form of an ultrasound image scan of a patient's abdomen to calculate the volume of urine in the patient's bladder, but this procedure can only produce approximate results and would not be suitable for venous reservoir evaluation.
There consequently remains a need for a system which can provide a substantially exact measurement of the fluid volume in a soft-shell venous reservoir on a continuing basis.